1. Field of the Invention
The present invention pertains to hermetically encapsulated devices such as an encapsulated infrared sensor, an encapsulated angular velocity sensor, and an encapsulated acceleration sensor. Especially, the invention relates to gettering units, which adsorb generated gaseous molecules in enclosed spaces of the encapsulated devices, being encapsulated with active elements such as these sensors in a package, under specific gas ambient or reduced pressure.
2. Description of the Related Art
An earlier gettering unit was implemented in a cylindrical geometry so as to achieve the heater built-in structure. And, there are some devices such as infrared sensors, which should be operated in vacuum environment, encapsulated in a hermetically sealed package. On a same stem, a sensing element, or a sensor chip, and the cylindrical gettering unit are mounted. A cap of the package is hermetically bonded to this stem by projection welding so as to encapsulate the cylindrical gettering unit. And, an infrared window made of germanium (Ge) or silicon (Si), etc. is bonded to the central portion of the cap, disposed at the upper part of the sensing element, with sealing glass. After the inside of the package is vacuum evacuated, a gettering material of the heater built-in gettering unit embedded in the package is activated, and the exhaust tube is sealed off so as to form a vacuum encapsulated package. As the gettering material is chemically active and removes the traces of gas remaining, or adsorbs the gaseous molecules generated in the vacuum after the vacuum encapsulation step completed, the enclosed space of the package is kept to be vacuum state.
To fabricate the heater built-in gettering unit having the cylindrical geometry, a central heater line is prepared. Next, this heater line is covered selectively with an insulating material such as aluminum (Al2O3) so as to form the cylindrical geometry by electrophoreses, etc, leaving two end terminals of the central heater line as lead wires. Then, the gettering material, in which graphite, etc. is mixed, is pressed so as to forge the cylinder on the insulating material, using top and bottom stamps. The gettering material cylindrically covers in a smaller area than the area that the insulating material surrounds the central heater line. Finally, the baking of this gettering material completes the heater built-in gettering unit.
Next, lead wires of the cylindrical gettering unit are connected with feedthrough lines of a stem by spot welding. The cylindrical gettering unit is installed in the package, by mounting the heater built-in gettering unit on the stem and connecting electrically the heater built-in gettering unit with circuitry disposed outside of the package. Then, a sensing element is mounted on the stem by die bonding. Then, bonding pads of the sensing element and feedthrough lines are electrically connected with bonding wires. Next, the infrared window is connected to the center of the cap by sealing glass. Then, the cap and the stem are bonded together by projection welding. Next, the exhaust tube of the package is connected with a vacuum pump through an exhaust conduit. An AC power supply is connected with the feedthrough lines in order to energize the heater built-in gettering unit. The gettering material is heated by the electric energy and is thermally activated, while the inside of the package is evacuated by the vacuum pump. Finally, the exhaust tube is sealed off so as to assemble the vacuum encapsulated package.
However, in such cylindrical gettering unit, the gettering material can not be activated uniformly, because the gettering material surrounds the heater line in the shape of the cylinder, and the temperature of the outside portion of the gettering material is lower than the inside portion.
Further, the gettering material of the inside, which is more activated at higher temperature than the outside, cannot sufficiently be submitted to act, since the gettering material of the inside is surrounded by that of the outside with the low activity. Therefore, the amount of gettering material much larger than the necessary and sufficient amount must be used.
An object of the present invention is to provide a gettering unit, which overcome the above-mentioned problems, and can implement uniform activation of the gettering material, and can reduce the quantity of the gettering material to the necessity minimum.
Another object of the present invention is to provide an integrated circuit merging an active element such as a sensor with the gettering unit, making uniform the activation of the gettering material, and reducing the quantity of the gettering material to the necessity minimum.
Still another object of the present invention is to provide an encapsulated semiconductor device mounting an active element such as a sensor with the gettering unit, making uniform the activation of the gettering material, and reducing the quantity of the gettering material to the necessity minimum.
Yet still another object of the present invention is to provide a method for manufacturing the gettering unit, making uniform the activation of the gettering material, and reducing the quantity of the gettering material.
First feature of the present invention inheres in a gettering unit encompassing (a) a substrate having top and bottom surfaces; (b) a thin film heater disposed on the top surface of the substrate; and (c) a gettering layer disposed on the thin film heater.
Second feature of the present invention inheres in a gettering unit encompassing (a) a first conductivity type semiconductor substrate having top and bottom surfaces; (b) a second conductivity type impurity doped region disposed at the top surface and in the single crystal substrate, the impurity doped region serving as a thin film heater, second conductivity type is opposite to the first conductivity type; and (c) a gettering layer disposed on the thin film heater.
Third feature of the present invention inheres in a semiconductor integrated circuit encompassing (a) a single crystal substrate having top and bottom surfaces, the top surface embracing a first area and a second area neighboring to the first area; (b) a thin film heater disposed selectively on the first area; (c) a gettering layer disposed selectively on the thin film heater.; and (d) an active element disposed in the second area.
Fourth feature of the present invention inheres in an encapsulated semiconductor device encompassing (a) a stem having plurality of feedthrough lines; (b) a active element chip mounted on the stem having plurality of bonding pads; (c) an auxiliary chip mounted on the stem, the auxiliary chip having top and bottom surfaces; (d) a thin film heater disposed on the top surface of the auxiliary chip having first and second end terminals; (e) a gettering layer disposed on the thin film heater; (f) a first set of bonding wires connecting between the bonding pads to corresponding feedthrough lines, respectively; (g) a second set of bonding wires connecting between the first and second end terminals to corresponding feedthrough lines, respectively; and (h) a cap bonded to the stem so as to encapsulate the active element chip and the auxiliary chip.
Fifth feature of the present invention inheres in a gettering unit encompassing (a) a thin film gettering means for adsorbing gaseous molecules in an enclosed space; (b) a thin film heating means for heating two dimensionally the gettering means so as to activate uniformly gettering action of the gettering means; (c) a supporting means for supporting the gettering means and heating means; and (d) a thermal isolation means for suppressing heat conduction from the heating means to outer environment.
Sixth feature of the present invention inheres in a method for manufacturing a gettering unit encompassing (a) depositing resistive material on a substrate; (b) delineating the resistive material so as to form a thin film heater; (c) depositing gettering material on the thin film heater; and (d) delineating the gettering material so as to form a gettering layer.
Seventh feature of the present invention inheres in a method for manufacturing a gettering unit encompassing (a) forming a first insulating film on a top surface of a substrate; (b) delineating a thin film heater on the insulating film; (c) forming a second insulating film on the thin film heater; (d) opening contact holes in the second insulating film so as to expose selectively top surface of the thin film heater; (e) forming surface interconnections connected to first and second end terminals of the thin film heater; (f) opening etching windows, by selectively removing the first and second insulating films so as to expose top surface of the substrate; (g) forming a gettering layer on the second insulating film; (h) selectively removing a portion of the substrate just under the gettering layer so as to form a cavity.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the present invention in practice.